1. Field of the Invention
The present invention relates to integrated circuits and, in particular, to cascode amplifier integrated circuits.
2. Description of the Related Art
Amplifier integrated circuits (ICs) constitute one of the basic types of analog integrated circuits. Amplifier ICs are essentially configured to receive an input voltage signal (Vin), or input current signal, and, in response, provide a larger output voltage signal (Vout), or larger output current signal. Such amplifier ICs can be used, for example, as an audio power amplifier or to drive a cathode ray tube (CRT).
FIG. 1 is a simplified schematic simulation diagram of conventional cascode amplifier IC 10. The conventional cascode amplifier IC 10 is configured to assert an amplified output voltage signal Vout (at output signal terminal 12) in response to input voltage signal Vin (received at input signal terminal 14), when biased by bias potential Vb (also referred to as a bias voltage and received at bias input terminal 16) and provided with power supply voltage Vcc (received at power supply input terminal 18).
Conventional cascode amplifier IC 10 includes an input buffer stage circuit 30 that includes bipolar transistor X8 and resistor R5, a gain stage circuit 32 that includes bipolar transistors X6 and X7 and resistor R2, an output buffer stage circuit 34 that includes interconnected bipolar transistors X0, X1, X2, and X3, and a bias stage circuit 36 that includes two diodes D1 and D2. In the conventional cascode amplifier IC 10 of FIG. 1, a stray capacitance (represented by dashed stray capacitor 1) is inherently present at node al3 of the output buffer stage circuit 34. Resistors R1a and R1b of conventional cascode amplifier IC 10 constitute a collector load resistor of the gain stage circuit 32.
One skilled in the art will recognize that FIG. 1 is representative of a variety of well known conventional cascode amplifier IC configurations. Further descriptions of cascode amplifier ICs are found in U.S. Pat. No. 5,977,610 to Hon Kin Chiu, P. R. Gray and R. G. Meyer, Analysis and Design of Analog Integrated Circuits, Third Edition, 225-226, 464-466 and 511-513 (John Wiley and Sons, 1993), and U.S. patent application Ser. No. 09/615,293, filed on Jul. 13, 2000, by Peyman Hojabri et al., entitled xe2x80x9cCascode Amplifier Integrated Circuit With Frequency Compensation Capability,xe2x80x9d each of which is hereby fully incorporated by reference.
A drawback of conventional cascode amplifier ICs is a slow transient fall response (i.e., a relatively long transient fall response time) due to a relatively slow discharging of the stray capacitance at a node of the output buffer stage (such as at node al3 of output buffer stage circuit 34 in FIG. 1). The relatively slow discharging of this node is worsened when the collector load resistance of the gain stage (e.g., resistors R1a and R1b in FIG. 1) is large (e.g, 6000 ohms). Still needed in the field, therefore, is a cascode amplifier integrated circuit that provides for a relatively fast transient fall response (i.e., a relatively short transient fall response time) and, therefore, relatively fast operation.
The present invention provides a cascode amplifier integrated circuit (IC) with a relatively fast transient fall response and, therefore, a relatively fast operation. A cascode amplifier IC according to the present invention includes a bias input terminal configured to receive a bias potential Vb, (e.g., a 12 volt bias voltage), a power supply input terminal configured to receive a power supply voltage Vcc (e.g., an 150 volt power supply signal), an input signal terminal configured to receive an input voltage signal Vin, and an output signal terminal. The cascode amplifier IC also includes a gain stage circuit and an output buffer stage circuit. The output buffer stage circuit includes a plurality of interconnected bipolar transistors. The gain stage circuit is configured to amplify the input voltage signal received at the input signal terminal and to, thereby, produce an amplified voltage signal, while the output buffer stage circuit is configured to receive the amplified voltage signal from the gain stage circuit, increase the current thereof and transmit the resultant amplified voltage signal with increased current to the output signal terminal as an amplified output voltage signal. The cascode amplifier IC further includes a discharge circuit configured to discharge stray capacitance at a node of the output buffer stage circuit. The discharge circuit includes, for example, a bipolar discharge transistor (configured, for example, as an emitter follower device) that reduces the Miller capacitance across the base and collector of one of the interconnected bipolar transistors of the output buffer stage circuit during a transient fall response. The discharging of the stray capacitance by the discharge circuit decreases the transient fall response time of the cascode amplifier IC and, thereby, increases its operation speed.